Pope Benedict XVI (#266) turned 83 on April 16, 2010.
When elected on April 19, 2005, he had just turned 78. The conclave that elected him started two days after his birthday.
At 78 and 3 days he was the 5th oldest to be elected pope (as on 1400).
[Dates pertaining to the popes prior to 1400 are either unreliable or unavailable and as such are impractical for meaningful analysis.]
Benedict XVI was the oldest to have been elected in 274 years.
The prior pope older than him was Clement XII (#247) who was elected on July 12, 1730. Clement, was also 78, but was 83 days older.
The oldest popes when elected, as of 1400, are:
1. Clement X (#240) who was 79 years 9 months when elected on April 29, 1670.
2. Alexander VIII (#242) who was 79 years 5 months when elected on October 6, 1689.
3. Paul IV (#225) who was 78 years 10 months when elected on May 23, 1555.
4. Clement XII (#247) who was 78 years 3 months when elected on July 12, 1730.
5. Benedict XVI (#266) who was 78 years 3 days when elected on April 19, 2005. The current pope.
[Note that Clement X and Alexander VIII were separated by just one pope, Innocent XI (#241) was 65 when elected.]
Alexander VIII papacy lasted 15 months while that of Paul IV was just over 4 years long. Clement X’s papacy was over 6 years long while that of Clement XII 9 and ½ years long.
The oldest pope since 1400 was Leo XIII (#257) who was 93 years, 4 months and 18 days old when he passed away on July 20, 1903.
The list of the oldest popes, as of 1400, is as follows (to June 18, 2010):
On July 19, 2010, Benedict XVI will overtake Gregory XIII to take over the 7th slot. At that point he would be in a ‘berth’ very familiar to him. Below that of his mentor John Paul II. John Paul II was 84 years and 10 months old when he died. So Benedict will not usurp him till 2012.
It is interesting that only 5 of the 11 are as of the 19th century [during which we had 15 popes].
Hope this helps.